Thursday, November 25, 2010

The emergence of an adult dragonfly from it's larval exoskeleton is one of the most remarkable events in the natural world. Within a short span of time the insect's body is transformed from a squat, cryptically colored aquatic form to a gaudy, streamlined aerial configuration. Thanks to Pauline Campbell for sharing her striking series of images documenting this metamorphosis. The photos were taken at Sand Lake, near Elgin, Ontario.

Dragonflies and damselflies go through incomplete metamorphosis, with their life cycle consisting of only three stages, the egg, the larva or nymph (or more properly, since the larva is aquatic, a naiad or water nymph), and the adult or imago. There's no dormant pupal stage between larva and adult as with butterflies, wasps, beetles and flies.

The first image should actually be last. It's the cast-off larval exoskeleton or exuviae; note the split in the thorax where the dragonfly has exited. However, this picture will serve to illustrate the dragonfly naiad. An insect's exoskeleton is rigid, and in order to be able to grow to this size the naiad has already undergone several molts. Each stage between molts is called an instar.

When it's time to undergo its final molt the naiad instinctively leaves its watery home and crawls to a nearby stone, stick or stem along the shoreline to begin the amazing transition to maturity.

Free at last! The newly emerged adult, called a teneral, pumps haemolymph (insect circulatory fluid) through its body, unfurling the crumpled, stubby wings and lengthening the abdomen. Needless to say, in this state the dragonfly is completely defenceless and vulnerable to predators.

The wings are now fully deployed. This dragonfly is a obviously clubtail, a member of the family Gomphidae, but the colors at this stage are so pallid that it's difficult to identify the species on this basis. Judging by the wide leaf-like profile of the exuviae and comparing it to this Dragonhunter naiad at Discover Life, and looking at this teneral Dragonhunter at BugGuide.Net, I'm going to hazard a guess that Pauline's dragonfly is probably a male Dragonhunter (Hagenius brevistylus).

Although the dragonfly's body and wings are still soft and delicate it's ready to take flight to the relative safety of the shoreline vegetation. In a few days the exoskeleton will harden and assume its adult coloration, and the fully mature insect will be ready to mate and reproduce, perpetuating a cycle of life that extends back 300 million years in time.